It is known that corrosion of amorphous carbon catalyst supports and metal catalyst of proton exchange, polymer electrolyte membrane (PEM) fuel cells, that occurs during high cell voltage conditions (e.g., above about 0.87V), results in a permanent decay of fuel cell performance. During shutdown, unless an inert gas purge is used, air will slowly, uniformly fill both the anode and cathode flow fields of the fuel cell. During startup, hydrogen is fed to the anode flow field which results in the inlet to the anode flow field being primarily hydrogen while the exit of the anode flow field is primarily air. This raises the potential of the cathode, opposite to the air rich zone on the anode, to a potential of 1.4-1.8 volts versus a standard hydrogen electrode. This potential causes the carbon based catalyst support to corrode and results in decreased cell performance.
In automotive applications, that may experience 50,000-100,000 startup/shutdown cycles, this results in catastrophic performance loss. Heretofore, solutions to this problem include stabilizing the fuel cell stack by purging the anode flow fields with an inert gas, such as nitrogen, and maintaining an auxiliary load across the fuel cell stack during the shutdown and startup processes. More recent solutions require maintaining a hydrogen rich gas throughout the stack (fuel and air) and associated plumbing.
In automotive applications, the availability of an inert gas, and the apparatus to employ it for purging will be prohibitively complex and expensive. The maintenance of a hydrogen rich gas will also require design complexity. The use of an auxiliary load requires dissipation of the heat generated thereby.
In automotive applications, PEM fuel cell power plants typically have a very wide range in demand, the swings to very low demand causing open circuit voltage conditions. Under open circuit voltage conditions, the high relative cathode voltage causes cathode catalyst corrosion, which in turn results in excessive performance decay. Because such fuel cells also have sudden increases in power demand, the reactant air flow to the cathode must be available to meet such demand, and therefore the air pump must continue to operate during low demand in order to accommodate a quick resumption of a higher demand for power.
In patent publication US 2006/0068249 A1, during a startup and shutdown or other power reduction transitions of a fuel cell stack, the spurious energy generated by the consumption of reactants therein is extracted in the form of electrical energy and stored in an energy storage device associated with the fuel cell power plant. Disclosed are a boost conFIGuration, useful when the voltage of the fuel cell stack is lower than the voltage at which it is desired to store energy in the energy storage device, and a buck configuration which is useful when the voltage of the stack is greater than the voltage at which energy is to be stored in the energy storage system.
In patent publication US 2009/0098427 A1, reactant air provided by a blower is rapidly diverted to ambient in response to low power demand that could result in high cathode voltage conditions, such as greater than 0.87 volts per cell. The blower is run at a higher level than required during the low output power demand and is thus ready to respond to rapid increases in output power demand.
An optional auxiliary load may be connected in parallel with the normal load whenever there is a rapid drop in output power demand, thereby dissipating the power which is generated in the process of consuming oxygen which remains in the stack, that is, residual oxygen in the flow fields and absorbed on the catalyst. The auxiliary load may be cooled by air from the blower which is diverted to ambient during low demand.
In certain automotive applications, such as parcel delivery and city passenger buses, as an example, the reduction of power demand to idle conditions is frequent. For a typical bus route, the bus will start only once or twice a day but will experience as many as 1200 stop/go cycles, where the bus slows down, the demand goes to an idle condition, and thereafter the bus resumes full power demand. It is therefore necessary to control cell voltages in an appropriate fashion for the different conditions of the fuel cell stack.